1. Field of the Invention
This invention relates generally to polyurethane compositions. More particularly, the invention relates to the use of moisture-curing, expanding polyurethane compositions as adhesives for fixing insulating materials to parts of buildings. The use according to the invention affords advantages in particular in the construction of lightweight flat roofs.
2. Statement of the Prior Art
It is known from German patent application No. 21 18 465 that polystyrene foam can be bonded to a sheet metal to form a roof covering, by means of an aqueous emulsion using a mineral fiber board intermediate layer permeable to water vapor. A disadvantage here is the long drying time of the emulsions which is also greatly affected by the air humidity level. Thus, in practice, additional mechanical fastenings such as nails or screws often have to be applied to prevent the insulating boards from being lifted by the effect of wind during the laying and before application of the final sealing layer.
It is also known that solvent-containing bitumen adhesives can be used for fixing insulating and other materials to parts of buildings. In this process, it also takes several hours for the bond to reach its ultimate strength. Another disadvantage, which also applies to emulsion-based adhesives, is the contraction in volume which the bond line undergoes during drying. Accordingly, to obtain reliable bonds, the adhesive has to be applied in comparatively thick layers, particularly where the surfaces to be bonded are uneven. This results in a relatively high consumption of adhesive.
Other disadvantages of the adhesives previously used are that emulsion-based adhesives are susceptible to condensation water while bitumen adhesives show poor temperature stability.
Polyurethane prepolymers have long been known. They are used for a variety of purposes in industry and are available in many different forms. Thus, it is known that one-component and two-component foams can be formulated on the basis of polyurethanes. Foams such as these are generally available, or may be employed, in aerosol form. Polyurethane foams of this type are described, for example, in U.S. Pat. No. 3,830,760, and corresponding German patent application No. 20 45 852. The products cure through the diffusion of moisture and are formulated in such a way that they undergo a marked increase in volume after application, which is desirable where they are used as insulating and filling materials per se. Because of their large-cell, loose structure, foams such as these never attain satisfactory mechanical strength levels and therefore cannot be considered for bonding-related applications. In addition, it is difficult to obtain uniform surface application from aerosol application.
On the other hand, polyurethane-based adhesives are also known. They either consist of prepolymers containing terminal isocyanate groups, which cure in the presence of moisture, or are formulated as twocomponent products where the prepolymers are mixed immediately before bonding with a curing agent, for example a polyhydric alcohol or a polyfunctional amine. Those concerned with polyurethane adhesives have made numerous attempts to suppress the formation of foam in order not to weaken the strength of the bond line. For example, numerous additives have been developed to absorb traces of water or reaction products thereof with a view to preventing foaming.
Thus, urethane foams are not suitable for use in accordance with the invention because they can only be applied as aerosols and because, on account of their large-cell structure, they form bonds which lack strength. Non-foaming polyurethane adhesives are also unsuitable because the application in question generally involves uneven surfaces which would lead to a high consumption of adhesive and hence to increased costs.